1. Field of the Invention
The present invention relates to a piezoelectric transformer inverter for converting direct current to alternating current by means of a piezoelectric transformer, and more particularly, to a piezoelectric transformer inverter which can be operated at a high efficiency and in a wide input voltage range, even if the load-current variation width is large.
2. Description of the Related Art
Back lit, liquid crystal displays have been commonly used as displays for portable information processors such as note-sized personal computers and the like. A fluorescent tube such as a cold-cathode tube or the like is generally used as a light source for the back light. Such tubes require a high voltage alternating current in order to turn them on.
Most portable information processors such as a notebook-sized personal computer or the like, use both a battery and an AC adapter as a power supply. Therefore, it is necessary to provide a DC/AC inverter or the like which can convert a low voltage direct current, supplied from the input power supply, to a high voltage alternating current capable of turning on the fluorescent tube.
In recent years, piezoelectric transformer inverters equipped with piezoelectric transformers which are smaller than electromagnetic transformers have been developed for use with such fluorescent tubes. For the above-mentioned uses, the piezoelectric transformer inverters typically need to have the following performance characteristics: the current (tube current) flowing through the fluorescent tube must be variably controlled in order to condition the brightness of the liquid crystal display; the inverter can be used in a wide input voltage range from the voltage of a power cell to that of a charger; and the conversion efficiency at which the direct current is converted to the alternating current must be high in order to prolong the time-period during which the tube can be powered by the power cell.